Many semiconductor processing applications are known wherein it is desirable to outdiffuse a dopant, into a semiconductor substrate, from a dopant source which subsequently functions as a contact to the doped substrate region. In many current applications relating to the formation of high-performance transistors, doped polysilicon is used both as the dopant source and as an ohmic contact to the diffused region. See, for example, U.S. Pat. No. 4,319,932 to Jambotkar (assigned to the assignee of the present invention), wherein polysilicon is used both as a diffusion source and contact for the extrinsic base region of a vertical, bipolar transistor. Polysilicon, however, has the disadvantage of exhibiting a relatively high sheet resistance.
U.S. Pat. No. 4,490,193 to Ishaq et al., assigned to the assignee of the present invention, shows a method of making a diffusion into a semiconductor substrate, using a rare earth boride dopant source, wherein the rare earth boride material remains as an ohmic electrical contact to the doped region. U.S. Pat. No. 4,481,046 to Bouldin et al., also assigned to the assignee of the present invention, is similar to Ishaq et al. in that the latter teaches the use of a rare earth hexaboride material, containing a predetermined amount of silicon, for the same purposes. These rare earth boride/hexaboride materials suffer from the disadvantage of reacting with silicon at temperatures in the 1,000+.degree.C. range. Such a reaction can damage the underlying silicon region or devices contained therein.
Tang, T., et al., "VLSI Local Interconnect Level Using Titanium Nitride," IEDM 85, 590-593 shows a method of forming L1 level interconnects using titanium nitride (TiN) formed incidentally to the fabrication of titanium silicide (TiSi.sub.2). More specifically, the TiN, which is incidentally formed over oxide regions during the TiSi.sub.2 process, is selectively masked and etched to form a first level interconnect material. The TiN does not function as a dopant source.
Roberts, S., et. al., "Method for Forming Shallow P+ Diffusions," IBM Technical Disclosure Bulletin, Vol. 30, No. 5, Oct. 1987, pg. 404, teaches the formation of shallow P+ diffusions in silicon using sputter-deposited titanium boride as a diffusion source. The titanium boride is subsequently removed.
U.S. Pat. No. 4,734,386 shows the use of chemical vapor-phase deposited boron nitride as a solid dopant source for diffusion doping of semiconductor substrates.